Flexible Ribbon Guide for In-Vivo and Hand-Held THz Imaging: Recent interest in terahertz frequency imaging for medical applications (wavelength range from 1 mm to 100 microns) has stimulated a flurry of new instrument proposals using both time domain and high resolution frequency domain spectral techniques. However, the field is very new, and definitive contrast mechanisms other than strong liquid absorption, have not yet been established. Terahertz studies on basal cell carcinoma, both in-vivo and ex-vivo, are very promising but the instrumentation employed is limited to fixed-position skin surface scans only. The problem lies with the fact that there exists no methodology for flexibly transporting terahertz signals from place-to-place with low loss, other than rigid-path free-space quasi-optical beam propagation. This means no terahertz instrument can be used in a hand-scanning mode or an in-vivo endoscopic application, with its greatly enhanced range of surveyable tissue and contrast. This research will change the modality and capability of all terahertz imaging instruments providing contrast that cannot be obtained today. In order to take advantage of techniques common at optical wavelengths, including in-vivo and portable hand-held sensor/receiver systems, the equivalent of signal-confining optical fiber links must be developed for the terahertz bands. Commonly employed transparent materials in the visible are all extremely lousy at longer wavelengths due to strong vibrational mode absorption. Dielectric substances with low absorption coefficient and high index do exist at terahertz frequencies, but they tend to be crystalline and therefore have poor mechanical properties when it comes to forming flexible guiding structures. Metallic waveguide (hollow or coaxial), although somewhat flexible, has untenable high resistive wall loss. A few plastics have very low dispersion and absorption but have a low refractive index that makes it difficult to confine single mode terahertz energy as it propagates around bends or through joints. Work by our collaborators has shown that high index materials formed into ultra-thin ribbons can form very low loss guiding media at millimeter-wave frequencies (30-300 GHz). Extrapolating this concept into the terahertz bands, and taking advantage of modern thin film fabrication techniques, we believe it is possible to use a combination of electro-deposited high-dielectric-constant crystalline materials in conjunction with low-loss, low index plastics to produce the equivalent of flexible terahertz optical fiber, i.e., "terahertz ribbon guide." The work to be performed will offer a solution to a major stumbling block in terahertz imaging and will significantly enhance the range of applications for all terahertz imagers, making it possible to scan fixed samples directly, or transport terahertz signals into the body where additional modalities, such as localized thermography, may be explored.